The venoms of five rattlesnakes are being studied for their chemical and physiopharmacologic properties. Fractions of the venom will be isolated, purified and characterized by gel filtration, ion exchange chromatography, qualitative and quantitative gel electrophoresis, ultra-filtration, ultracentrifugation, amino acid analysis and, in the case of peptides, amino acid sequencing. The LD 50 of each fraction will be determined and those fractions which prove more lethal than the crude venom, or have relatively similar LD 50 values, will be studied for their effects on nerve-muscle preparations, the central nervous system, and the cardiovascular system. Studies will also be made on certain coagulant-anti-coagulant properties of the venom fractions, as well as on immunological phenomena. Consideration will be given to the chemical and pharmacological evolution of those venom fractions that appear to have a specific affinity for the nervous system, while attempting to correlate this evolution with the anatomical changes that occurred in the venom apparatus. The use of these fractions as tools in the study of biological mechanisms and in clinical therapeutics will be determined.